1. Field of the Disclosure
The present disclosure relates to an image display apparatus including a holographic optical element (hereinafter, also referred to as an “HOE”).
2. Description of the Related Art
Image display apparatuses, such as a three-dimensional display, a head mounted display (HMD), and a head-up display (HUD), based on holography have been developed.
As an example of image display apparatuses used as HMDs, Japanese Unexamined Patent Application Publication No. 2010-271554 discloses an HOE display apparatus (image display apparatus) 909 including HOEs illustrated in FIG. 6A. FIGS. 6A and 6B are diagrams explaining the HOE display apparatus 909 as Related Art 1. FIG. 6A illustrates the configuration of the HOE display apparatus 909. FIG. 6B explains light emerging from a liquid crystal display (LCD) 913. As illustrated in FIG. 6A, the HOE display apparatus 909 includes an image display unit 901, the HOEs, i.e., an HOE 914 for display and an HOE 915 for detection, a brightness sensor (brightness detection unit) 916 to detect the brightness of sample light from the detection HOE 915, and a brightness adjustment unit 903. The image display unit 901 includes a light emitting diode (LED) 911, serving as a light source, the LCD 913, serving as a spatial modulation element, and an LCD driver 912. The brightness adjustment unit 903 includes an LED driver 918 and a digital signal processor (DSP) 917 to control the LED driver 918.
Referring to FIGS. 6A and 6B, the HOE display apparatus 909 is configured such that image light of the light emerging from the LCD 913 is guided to the display HOE 914 (refer to FIG. 6B) and sample light of the light is guided to the detection HOE 915 (refer to FIG. 6B). The sample light from the detection HOE 915 is detected by the brightness sensor 916. The LED driver 918 can adjust the power of the LED 911 based on the result of detection. The image light output from the image display unit 901 reaches a viewer 905, so that an image based on the image light obtained by adjustment can be visually identified by the viewer 905.
Japanese Unexamined Patent Application Publication No. 2010-271554 further describes a typical image display apparatus including no holographic optical element. FIG. 7A illustrates such an image display apparatus 700 as Related Art 2. As illustrated in FIG. 7A, the image display apparatus 700 includes an image display unit 701 including a light source, a brightness detection unit 702 to detect the brightness of sample light, and a brightness adjustment unit 703 to adjust brightness in the image display unit 701. Accordingly, the brightness adjustment unit 703 can adjust the intensity of light emitted from the image display unit 701 based on a result of detection by the brightness detection unit 702 so that the light intensity approaches a predetermined target value at which an optimum color balance is achieved. Image light emitted from the image display unit 701 reaches a viewer 705, so that an image based on the image light obtained by adjustment can be visually identified by the viewer 705.
Japanese Unexamined Patent Application Publication No. 2010-271554 further describes a typical image display apparatus including a holographic optical element. FIG. 7B illustrates such an image display apparatus 800 as Related Art 3. As illustrated in FIG. 7B, the image display apparatus 800 includes an image display unit 801 including a light source, a holographic optical element (or hologram optical element) 804 on which image light from the image display unit 801 is incident, a brightness detection unit 802 to detect the brightness of sample light from the image display unit 801, and a brightness adjustment unit 803 to adjust brightness in the image display unit 801. Accordingly, the brightness adjustment unit 803 can adjust the intensity of light emitted from the image display unit 801 based on a result of detection by the brightness detection unit 802 so that the light intensity approaches a predetermined target value at which an optimum color balance is achieved. Image light diffracted by the holographic optical element 804 reaches a viewer 805, so that an image based on the image light obtained by adjustment can be visually identified by the viewer 805.
As discussed in Japanese Unexamined Patent Application Publication No. 2010-271554, there is little difference in intensity between the image light and the sample light from the image display unit 701 in the image display apparatus 700 including no holographic optical element illustrated in FIG. 7A. The intensity of light from the image display unit 701 is adjusted based on the result of detection of the sample light without any problems. In the image display apparatus 800 including the holographic optical element illustrated in FIG. 7B, however, there is a large difference in intensity between the image light and the sample light because the image light leaving the holographic optical element 804 is diffracted light. It is significantly difficult to adjust the intensity of light from the image display unit 801 based on the result of detection of the sample light.
In the HOE display apparatus 909 according to Related Art 1 disclosed in Japanese Unexamined Patent Application Publication No. 2010-271554, the detection HOE 915 is disposed on an optical path of the sample light and the intensity of light from the image display unit 901 is adjusted based on the sample light from the detection HOE 915. Since each of the image light and the sample light is light leaving the holographic optical element, there is little difference in intensity between the image light and the sample light. Thus, appropriate adjustment may be achieved.
The area of an image displayed on a display screen is inversely proportional to the brightness of the displayed image. If the area of each display image component included in a displayed image varies from image to image, the brightness of the display image component significantly varies depending on the area thereof in the method of adjusting the intensity of light as in Related Art 2 or Related Art 3.
In the HOE display apparatus 909 according to Related Art 1, each of the image light and the sample light is light leaving the holographic optical element. As illustrated in FIG. 6B, however, the image light distinctly differs from the sample light in, for example, the direction of traveling and an image projected based on the light. Japanese Unexamined Patent Application Publication No. 2010-271554 discusses that the intensity and wavelength of the sample light, which is diffracted light, reaching the brightness sensor 916 differ from those of the image light reaching the viewer 905. Accordingly, the brightness of an image displayed on a display screen of the HOE display apparatus 909 according to Related Art 1 significantly varies depending on the area of the image. Disadvantageously, this results in significant unevenness in brightness of the image to be visually identified.